Glaciers & Ice Ages
 

Louis Agassiz, a Swiss geologist, recognized that glaciers explained erratic boulders. He observed that glaciers were powerful agents of landscape change.


Glaciers carried sand, mud, and huge boulders long distances and dropped these materials when they melted.
 

 

 

 

Agassiz proposed that ice sheets had once covered Europe, had sculpted the landscape, and had retreated, leaving behind fine-grained unsorted soils and erratic boulders.

 

Agassiz’s idea was criticized for decades, but finally accepted; the evidence was simply too strong. Agassiz saw evidence for an ice age in North America, too.
 

 

 

 

 

Recent Glaciations

Glaciers presently cover ~10% of Earth. During ice ages, this coverage expands to ~30%.
 

The most recent ice age ended ~11 Ka. New York, Montreal, London, and Paris were buried below hundreds to thousands of meters of crushing, grinding ice.
 

 

 

 

 

 

Ice and the Nature of Glaciers

 

What is ice?

  • Ice is solid water (H2O) that grows as hexagonal crystals when water cools below the freezing point.
  • Natural ice is a mineral that forms many types of rock.
  • Igneous ice crystallizes from a melt.
  • Sedimentary ice, such as falling snow, accumulates under the influence of gravity.
  • Metamorphic ice is deformed by plastic flow. Ice moves through the rock cycle quickly.
     

 

 

 

 

Snow falls like sediment and accumulates in layered strata.

Layers of snow recrystallize to become metamorphic glacial ice.
 

 

 

 

 

 

Conditions Needed for Glacier Formation

  • A cold local climate (which requires polar latitudes or a high elevation)
  • Snow must accumulate; more snow must fall than melts.
  • Snow must not be removed by avalanches or wind.
     

 

 

 

 

 

How a Glacier Forms

Under a microscope, glacial ice has coarse grains and contains air bubbles.

Air content decreases with age and degree of metamorphism.

Snow compacts and melts to form firn, which recrystallizes into ice.

Crystal size increases with depth.
 

 

 

 

 

 

 

 

Categories of Glaciers

  • mountain glaciers
  • continental glaciers

 

 

 

Mountain Glaciers

  • Mountain settings include a variety of glacier types that are classified based on shape and position.
  • Cirque glaciers fill mountaintop bowls.
  • Valley glaciers flow like rivers down valleys.
  • Mountain ice caps cover peaks and ridges.
  • Piedmont glaciers spread out at the end of a valley.
     

 

 

 

 

 

 

 

 

 

Piedmont glaciers spread out at the base of a mountain valley.
 

 

 

 

 

 

 

Continental Glaciers

Two major continental ice sheets exist today—Antarctica and Greenland.
 

 

 

 

 

 

 

 

Crevasses

  • Crevasses form in the brittle upper layer of a glacier.
  • Below ~ 60 m ice is a ductile plastic.
  • Cracking often takes place where the glacier bends while flowing over steps or ridges in its substrate.
     

 

 

 

 

 

 

 

 

The Movement of Glacial Ice

Ice flows downhill under the influence of gravity. The ice base can flow up a local incline.
 

 

 

 

 

 

Ice flows away from the thickest part of continental glaciers; analogous to honey flowing away from the thickest part of pile.
 

 

 

 

 

 

 

The Movement of Glacial Ice

  • Flow velocities vary with location in a glacier.
  • Overall, ice flows from the zone of accumulation to the toe.
  • Flow velocity is greatest in the center of the glacier.
  • Velocity decreases at the ice margins due to friction with the substrate.
     

 

 

 

 

 

 

 

  • Glaciers form when buried snow lasts all year and turns to ice.
  • Mountain glaciers form at high elevation and flow down slope. Ice sheets form in high latitudes and spread over continents.
  • Flow of ice can be accomplished by plastic deformation, or by basal gliding.

 

The equilibrium line separates the zone of accumulation from the zone of ablation.

Ice flows downward in the zone of accumulation and upward in the zone of ablation.

 

 

 

 

 

 

Glacial Advance and Retreat

The position of the toe represents a balance between addition by accumulation and loss by ablation.
 

 

 

 

 

 

If accumulation exceeds ablation, the glacier advances, the toe moves farther downvalley, and the ice thickens.

 

 

 

 

 

 

 

If ablation exceeds accumulation, the glacier retreats and thins. The toe moves upvalley, even though ice continues to flow toward the toe.


 

 

 

 

 

Ice in the Sea

Marine glaciers are grounded in shallow water and float in deeper water. Floating ice is mostly (80%) beneath the waterline. Icebergs form when the leading edge of the glacier breaks away (calves).
 

 

 

 

 

 

 

 

 

 

Floating ice exhibits a variety of shapes and sizes. Icebergs are >6 m above water. Ice shelves produce large tabular bergs.

Calved icebergs are laden with sediments eroded by the glacier and incorporated into the ice.

Icebergs raft sediments away where they are eventually released by melting.

Dropstones in marine muds are an indication of glaciation.
 

 

 

 

 

 

 

 

 

 

 

Sea ice is non-glacial ice formed from frozen seawater. Large areas of polar seas are covered with ice.
 

 

 

 

 

 

 

 

 

 

 

 

Glacial Erosion

Glacial erosion produces deep, steep-sided valleys and jagged, knife-edged ridges and pointed spires.
 

 

 

 

 

 

 

 

 

 

 

 

Rock fragments embedded in glacial ice act like sandpaper on underlying bedrock. The moving ice abrades and polishes substrates, producing a fine pulverized “rock flour.”
 

 

 

 

 

 

 

 

Roche Moutonnée

A roche moutonnée is an asymmetric bedrock hill shaped by the flow of glacial ice. Abrasion rasps the upstream side, and plucking carries away fracture bounded blocks on the downstream side.
 

 

 

 

 

 

 

 

 

 

Glacial Erosional Landscapes

Before glaciation, valleys are V-shaped, and tributary mouths are the same elevation as the trunk stream.
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Horns

A horn is a pointed mountain peak formed by three or more cirques that coalesce.
 

The Matterhorn in Switzerland
 

 

 

 

 

An arête is a knife-edge ridge formed by two cirques that have eroded toward one another.

A cirque is a bowl-shaped basin formed at the uppermost portion of a glacial valley.

Aggressive freeze-thaw chews into the cirque headwall.

After the ice melts, a cirque is often filled with a tarn lake.
 

 

 

 

 

 

 

Glacial erosion creates a distinctive U-shaped trough valley.

These are easily discerned from V-shaped fluvial valleys.
 

 

 

 

 

 

 

 

Hanging Valleys

A hanging valley results from the intersection of a tributary glacier with a trunk glacier.
 

The larger trunk glacier incises much deeper into the bedrock than the smaller tributary glacier.

When the ice melts, the troughs have different elevations and a waterfall results.

 

 

 

 

 

 

 

 

 

Fjords

Fjords are U-shaped glacial troughs that have become flooded by the sea.
 

 

 

 

 

 

 

 

Glacial Deposition: Transport of Sediment by Ice

Glaciers act as large-scale sediment conveyor belts. Sediment falls onto a glacier and gets plucked up from below. This material is transported to the toe, where it piles up as an end moraine

 

 

 

 

 

 

Glaciers are dirt machines; they carry an enormous amount of sediment.
 

 

 

 

 

 

 

 

 

Lateral Moraines

Lateral moraines form along either side of a valley glacier. Medial moraines occur in the middle of a valley glacier and result from the merging of two lateral moraines.
 

 

 

 

 

 

 

Glacial Till

Glacial till is unsorted, unstratified sediment dropped by glacial ice. Till is made up of all grain sizes, from boulders to clay. It accumulates beneath glacial ice, at the toe of a glacier and along glacial flanks.
 

 

 

 

 

 

 

 

 

Types of Glacial Sedimentary Deposits

 

Erratics

Glacial erratics are cobbles and boulders that have been dropped by a glacier, often on glacially polished bedrock.

 

 

 

 

 

 

Outwash is dominated by sand and gravel that have had the muds removed. Grains are graded and stratified, abraded and rounded. Glacial outwash is sediment transported by meltwater.
 

 

 

 

 

 

 

 

Loess

Loess (pronounced “luss”) is wind-transported silt. Glaciers produce abundant amounts of fine sediment, which is picked up and carried downwind.
 

 

 

 

 

 

Glacial lakes accumulate fine rock flour that settles out of suspension in deep lakes.

 

Glacial lake sediments often display seasonal varve layers that reflect the finest silt and clay from frozen winter months interlayered with coarser silt and sand from summer months.
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Depositional Landforms

 

Glacial sediments create distinctive landforms. These include end moraines, terminal moraines, recessional moraines, drumlins, ground moraines, kettle lakes, and eskers.
 

 

 

 

 

 

Cape Cod, Nantucket, Martha’s Vineyard, Block Island, Long Island, and other prominent landforms in northeastern United States formed at the end of the continental ice sheet.
 

 

 

 

 

 

Drumlins

Drumlins are elongate, tapered, and aligned hills of molded glacial till that formed underneath the continental ice sheet. They have an asymmetric form—steep up-ice, tapered down-ice—and are common as swarms aligned parallel to ice-flow directions. They are likened to a “field of swimming whales.”
 

 

 

 

 

 

 

 

 

 

Kettle Lakes

Ice blocks calve off of glaciers and become buried in sediment. When the ice melts, a kettle forms. Hummocky knob-and-kettle topography typifies ground moraine.
 

If the water table is high, the kettle will fill to become a kettle lake.
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Eskers

Eskers are long, sinuous ridges of sand and gravel. They form as meltwater channels within or below ice. Channel sediment is released when the ice melts.

 

 

 

 

 

The balance of accumulation and ablation determines whether a glacier advances or retreats.
 

Glaciers form distinctive landforms, such as U-shaped valleys, cirques, and horns. U-shaped valleys that fill with water become fjords.
 

Deposition by glaciers produces distinctive landforms, such as moraines, eskers, and kettle holes.
 

 

 

 

 

 

 

 

 

Continental Glaciation: Ice Loading and Rebound

Ice sheets depress the lithosphere into the mantle.

Slow crustal subsidence follows flow of asthenosphere.

This process continues slowly today. After ice melts, the depressed lithosphere rebounds and the land rises.
 

 

 

 

 

 

 

Sea Level Changes

Ice ages cause sea level to rise and fall because water is stored on land. Sea level was ~100 m lower during the last ice age. Deglaciation returns water to the oceans and sea level rises. If ice sheets melted, coastal regions would be flooded.
 

 

 

 

 

 

 

 

Sea level rise between 17 Ka and 7 Ka was the result of deglaciation.

Low sea level during the last ice age exposed continental shelves.

Prehistoric people migrated to North America from Asia via the Bering land bridge.
 

 

 

 

 

 

 

 

 

Ice Dams

Glacial Lake Agassiz covered 250,000 km2 (100,000 mi2) and existed for 2,700 years. It drained abruptly when the ice dam failed.
 

 

 

 

 

 

 

 

Pluvial* Features

Large lakes occupied today's Basin and Range deserts. The Great Salt Lake is a small remnant of the much larger Lake Bonneville. Weather patterns were different during glaciation; the American southwest was much wetter.
 

*Pluvial: a time period characterized by relatively high precipitation

 

 

 

 

 

 

 

 

 

The Pleistocene Ice Age: Glaciers

During the Pleistocene, several distinct ice sheets formed. In several places, neighboring sheets came into contact.
 

 

 

 

 

 

 

 

 

The Pleistocene Ice Age: Life and Climate

All climate and vegetation belts were shifted southward.

Tundra covered parts of the United States, and southern states had forests like those in New England today.

Cold-adapted, now extinct, large mammals roamed regions that are now temperate.
 

 

 

 

Pleistocene megafauna

Pleistocene megafauna is the set of large animals that lived on Earth during the Pleistocene epoch and became extinct during the Quaternary extinction event.

Megafauna is a term used to describe an animal with an adult body weight of over 44 kg.

 

 

During the American megafaunal extinction event around 12,700 years ago, 90 genera of mammals weighing over 44 kilograms became extinct.

 

 

 

 

 

 

The Pleistocene Ice Age: Timing

Oxygen isotope ratios from marine sediments define 20 to 30 Pleistocene glaciations. Earth history has witnessed many ice ages.
 

 

 

 

 

 

 

 

 

 

Causes of Ice Ages

Milankovic hypothesized that climate variation over 100 to 300 Ka is predicted by cyclic changes in orbital geometry.

Earth’s axis wobbles (precession) with a 23,000-year periodicity.

The angle of Earth’s rotational axis (obliquity) changes with a 41,000-year periodicity.

The shape (eccentricity) of Earth’s orbit around the Sun varies with a 100,000-year periodicity. Ice ages may result when cooling effects coincide.
 

 

 

 

 

 

 

Earth precession video

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Long-Term Cenozoic Cooling

 

Geologists have reconstructed an approximate record of global climate for geologic time. Over the last 100 million years, Earth experienced a warm climate at the end of the Mesozoic and climate cooling since the Oligocene.
 

 

 

 

 

 

 

 

 

 

 

 

Will There Be Another Glacial Advance?

 

Are we living in an interglacial? Will ice return? Very likely. Interglacials last ~10,000 years. It has been ~11,000 years since the last one. A cool period (1300 to 1850) resulted in the Little Ice Age in Europe. Today, a warming trend has caused glaciers to recede. Earth’s climate changes without consulting humanity.


 

 

 

 

 

 

Little Ice Age in Europe from about 1300 to about 1850.

 

 

 

 

 

The weight of a growing continental ice sheet causes underlying lithosphere to subside, then melting allows it to rebound.
 

During the Pleistocene (2.6 Ma to 11 Ka), ice sheets advanced and retreated up to 30 times.
 

Advances and retreats during a given ice age are controlled by Milankovitch cycles.
 

 

 

 

 

 

Melting Glaciers

 

 

 

 

 

 

Greenland Ice Sheet